ci: Work around broken python UTF8 handling in the CI docker image

Gitlab very kindly exposes the details of the git commit message (among
much else) in the environment. Unfortunately, piglit tries to handle the
environment in non-UTF8-safe ways, which means if the top-of-tree commit
mentions non-ASCII characters (say, in the author's name) then all the
tests fail and so does the pipeline.

Fortunately none of those variables are things our piglit invocation
needs. Since I've failed to rebuild the docker image as yet, just clear
the likely variables from the environment before running piglit.

This-makes-me: 
Status Job ID Name Coverage
  Test
passed #61192
job

00:00:57